(1) Field of the Invention
This invention relates to an image processing apparatus and an image processing method for a printing apparatus having at least two print heads spaced from each other in a transport direction of a printing medium. The image processing apparatus and image processing method are provided for processing print data to be printed by the printing apparatus.
(2) Description of the Related Art
Conventionally, as this type of apparatus there exists an inkjet printing apparatus including a transport path for transporting web paper, a printing unit disposed on the transport path for dispensing ink droplets to the web paper to form images thereon, and an image processing apparatus for outputting print data to the printing unit (see Japanese Unexamined Patent Publication No. 2013-176868, for example). In this apparatus, the printing unit includes three print heads arranged at intervals in a transport direction of web paper.
In the inkjet printing apparatus of such construction, when the web paper is transported along the transport path, the web paper may make a skew. When printing is done in this condition, the positional relationship between ink droplets dispensed from an upstream print head and ink droplets dispensed from a downstream print head will produce a positional deviation (also called a misregister), which is a deviation from a positional relationship intended in image data. Then, the image processing apparatus carries out a correction by shifting the image data in a width direction of the web paper according to an amount of skew, and outputs corrected image data to the printing unit. And the printing unit performs printing based on the corrected image data.
However, the conventional example having such construction has the following problems.
The conventional apparatus carries out a shifting process according to an amount of skew on a nozzle-by-nozzle basis which is a recording limit of the printing unit. It is therefore impossible to perform a sufficient correction, depending on an amount of skew which does not make an integral multiple of nozzle interval, thereby failing to fully inhibit a positional deviation. A failure to fully inhibit the positional deviation will cause a lowering of print quality. Thus, the inhibiting of the positional deviation is a serious issue.
A similar problem may arise from a cause different from the above skewing. That is, the above inkjet printing apparatus uses a rotary encoder attached to a transport roller to detect a transport speed of the web paper, and adjusts the timing of dispensing ink droplets according to the transport speed and a distance between the print heads. However, the transport roller may expand in the circumferential direction due to heat, for example. In such a state, the transport speed, although constant, will be detected in error, resulting in a positional deviation in the transport direction. In this case also, as in the case of skew, the shifting process on the nozzle-by-nozzle basis which is the recording limit may prove insufficient.